leap second

In all of our concerns about being on time, let us not forget that there is a debate, not a very loud one, but an old and persistent one, over how to measure it. Those with their heads in the clouds would prefer to measure it as the world turns; earth-bound precisionists want to measure a second as the more dependable duration of 9,192,631,770 cycles of an atom of the isotope cesium-133. To keep the two times in sync--the world is turning slower, and our days longer, by about 2 milliseconds per century--a leap second is added to atomic time every so often. One was just slipped in just before Jan 1, 2006. But like me you probably didn't notice the clock skip a beat because you were drinking.

Astronomers prefer to calibrate their telescopes, satellites, and other instruments against deep-space objects such as pulsars, which emit pulses of energy at regular intervals.

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The International Telecommunication Union decided that astronomical time could not differ from Coordinated Universal Time—which is based on atomic time—by more than 0.9 second. Because the two systems are inherently out of step, it’s periodically necessary to add “leap second” to bring them into sync. Most people didn’t notice, but one of those seconds was added after midnight on Dec. 31, 2005, just before 2006 began. A leap second, says Jonathan Betts of the Royal Observatory, “asks the atomic clocks to hold their breath for one second, so that the Earth can catch up.” So far, the compromise has worked. But some American scientists have proposed scrapping leap seconds altogether.

http://www.theweekmagazine.com/article.aspx?id=1295